As a positive type photoresist composition, a composition containing an alkali-soluble resin and a naphthoquinone diazide compound as a light-sensitive substance is generally used. For example, a composition comprising a novolak type phenol resin and a naphtho-quinonediazide-substituted compound is described in, for example, U.S. Pat. Nos. 3,666,473, 4,115,128 and 4,173,470, and a composition comprising a novolak resin of cresol-formaldehyde and trihydroxybenzophenone-1,2-naphthoquinonediazidesulfonic acid estesr, which is most typical, is described in L. F. Thompson, Introduction to Microlithography, ACS Publishing Co., Ltd., No. 219. pp. 112-121.
A novolak resin as a binder is particularly useful because it is soluble in an aqueous alkali solution without swelling and in using the formed image pattern as a mask for etching, exhibits high resistance especially against plasma etching. A naphthoquinonediazide compound as a light-sensitive substance acts as a dissolution-preventing agent which reduces the solubility of the novolak resin, but when decomposed by irradiation with light, produces an alkali-soluble substance and increases the alkali solubility of the novolak resin. Because of this large change in properties with light, the naphthoquinonediazide compound is particularly useful as a light-sensitive substance for the positive-type photoresist.
From the above viewpoint, a number of positive type photoresists containing a novolak resin and a naphthoquinone diazide-base light-sensitive substance have been developed and put into practical use, and satisfactory results have been obtained in the fabrication of line widths up to about 1.5 to 2 .mu.m.
With an increase in the degree of integration in integrated circuits, the fabrication of fine patterns having a line width of not more than 1 .mu.m has been needed in the production of substrates for semiconductors such as super LSI and so on. In this application, there is needed a photoresist which has a high resolving power, a high pattern reproducing accuracy permitting accurate copying of the shape of an exposure mask, and a high sensitivity from the viewpoint of high productivity, and further which is excellent in storage stability.
An attempt to increase a quinonediazide light-sensitive group content has been made to obtain a high resolving power, JP-A No. 60-158440, for example, describes a method using a light-sensitive substance having a high triester content. (The term "JP-A" as used herein means an "unexamined published Japanese patent application" .) JP-A No. 61-118744 describes an attempt to increase the number of quinonediazide light-sensitive groups contained in the molecule of a light-sensitive substance.
These light-sensitive substances, however, are insoluble in commonly used solvents, or even if soluble, form a precipitated material during the storage of the photoresist composition, causing problems at the photofabrication step or device fabrication step and a reduction in yield. In order to prevent the precipitation of a light-sensitive substance, JP-A No. 61-260239 describes a method of adding organic solvents having a solubility parameter of 11 to 12. However, since a solvent such as dimethylsulfoxide is used, the storage stability of the resist composition is poor, and sensitivity and resolving power markedly vary with a lapse of days. Thus the above photoresist cannot meet the requirements as described above.